In recent years, improvements in laser performance have resulted from the fabrication of so-called "superlattice" and quantum-well structures. A lattice is formed of alternate layers of thin (5-40 nm) epitaxial films of semiconductor material of different band gap to form a plurality of heterojunction interfaces. A thin film of narrow band gap material, e.g., GaAs, is sandwiched between thin films of wider band gap material, e.g., Al.sub.x Ga.sub.(1-x) As to form potential wells in the conduction and valence bands. Such wells restrict or limit carrier/electron movement to two dimensions. Such a multilayer is referred to as a two-dimensional quantum-well, or simply a quantum-well. In the example given, the electron motion is restricted in the direction perpendicular to the heterojunction interfaces, while the electrons are free to move in the other two directions.
Quantum-well heterostructures generally exhibit more efficient luminescence intensities than bulk crystal heterostructures and have, therefore, been incorporated into the active region of laser devices.
Devices are usually formed of multiple layers of quantum-wells and when the barrier thickness between quantum-wells is reduced, so that well wave functions couple, a "superlattice" is formed.
Generally, the materials of choice for superlattice and/or quantum-well structures are the Group III and V elements and, in particular, GaAs and Al alloys thereof. These materials are closely lattice matched, yet the difference in the band gaps of the GaAs versus the alloy Al.sub.x Ga.sub.(1-x) As can vary at room temperature from 0 eV to as much as 0.75 eV (as x increases from 0 to 1). The first property simplifies heterostructure fabrication, while the second property makes fabrication of quantum-wells possible.
GaAs based quantum-well devices are limited in wavelength range from 0.7 to 0.9 microns. Thus, for certain applications, researchers have turned their attention to other materials. In particular, since 1984, lasers operating at the wavelength of 2-4 microns have been sought for use in long haul communication over fluoride glass fibers. The then current silica fibers utilized a wavelength of 1.55 microns, but were much more lossy than the fluoride glass fibers were predicted to be at a wavelength of 2-4 microns.
Based upon this need, research was directed toward development of GaInAsSb compound structures, since this alloy has a room temperature direct band gap which is continuously adjustable between 0.29 and 0.73 eV and which corresponds to a wide spectral range between 1.7 and 4.3 microns.
Despite the need for such a laser device using this alloy, Chiu et al. report in a 1986 article, "Room-Temperature Operation of InGaAsSb/AlGaSb Double Heterostructure Lasers Near 2.2 .mu.m Prepared by Molecular Beam Epitaxy", Appl. Phys. Lett., 49(17), 27 Oct. '86, pp. 1051-1052, that studies of the growth of this alloy were scarce and no optical device work had been reported.
Chiu et al. in the referenced article report a room temperature GaInAsSb/AlGaSb injection laser using a molecular beam epitaxy (MBE) grown double heterostructure.
The Chiu et al. laser, however, exhibited poor optical and electrical confinement and crystal defects that degraded laser performance. The lattice mismatch between the Al.sub.0.35 Ga.sub.0.65 Sb and GaSb materials used by Chiu et al. is 2.3.times.10.sup.-3. The so-called "critical layer thickness," or the maximum layer thickness that can withstand this lattice mismatch before dislocations are formed, is in the range 100-200 nm. Since the layers of Chiu et al. are much greater than this (two layers of 3 .mu.m each), one can reasonably surmise that a large number of dislocations were formed.
Thus a need exists for a high performance AlGaAsSb laser and, more particularly, for such a laser formed with an advanced more complex quantum-well structure and with minimum crystal defects.